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Instability-driven electromagnetic fields in coronal plasmasa)
a)Paper QI3 3, Bull. Am. Phys. Soc. 57, 289 (2012).
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Image of FIG. 1.
FIG. 1.

(a) Experimental setup used to radiograph plasma evolution of solid CH spheres irradiated with an intensity of ∼2 × 10 W/cm. (b) Summary of resultant proton radiographs taken during the 1 ns pulse. Darker pixels indicate higher proton fluence, but the gray scale is different in each image. (c) Profiles of the fluid, Nernst, and advection velocities calculated from 1-D radiation-hydrodynamic simulations at 1 ns. Radial distances are given relative to the ablation front and positive velocities are pointed radially outward.

Image of FIG. 2.
FIG. 2.

A schematic drawing of the experimental setup used to radiograph directly driven plastic foils using (a) protons and (b) x rays. Proton and x-ray images were recorded on CR-39 and film, respectively.

Image of FIG. 3.
FIG. 3.

Summary of proton-fluence radiographs taken of irradiated plastic foils with different initial surface perturbations. Radiographs were taken during the last nanosecond of the laser drive and after the end of the pulse. Images have been individually normalized so that the grayscale ranges from ±30% of the mean fluence in each radiograph. Cellular structures are observed in radiographs for times irrespective of initial foil perturbations.

Image of FIG. 4.
FIG. 4.

(a) Dominant scale size of cellular features observed in proton radiographs as a function of time. For times , no characteristic size was measured. (b) Normalized broadband rms amplitude of proton fluence as a function of time. The rms amplitude is shown to grow in time similarly for all foil types and continue after the laser drive has ended.

Image of FIG. 5.
FIG. 5.

(a) Summary of sample x-ray radiographs of the four different foil types. Images were taken during and after the laser drive as indicated by the image location relative to the pulse schematic. Those foils with preimposed surface modulations demonstrate RT-growth of the seeded perturbations. (b) Analysis of the 120 m data show good agreement with radiation-hydrodynamic predictions of RT-growth.

Image of FIG. 6.
FIG. 6.

X-ray images taken at ∼2.2 ns of both flat and modulated foils are shown on the left. Lineout directions are indicated by arrows in the images, where spectra from multiple lineouts were averaged for the flat foil case and lineouts parallel (Mod) and perpendicular (no Mod) to the perturbation wave vector are shown for the 120 m case.

Image of FIG. 7.
FIG. 7.

(a) Predicted DRACO profiles of electron temperature and density taken at 1.5 ns. Major field-generating instabilities are listed and the locations where they occur in the plasma are indicated. The ablation, critical, and quarter-critical surfaces are labeled for reference. The absolute value of the axial position is arbitrary and a log scale is implemented to show the small-scale variation near the ablation front. The Nernst velocity changes direction at the peak temperature just outside the quarter-critical surface. (b) Profiles of the fluid, Nernst, and advection velocities calculated from DRACO simulations. Distances are given relative to the ablation front and positive velocities are pointed outward.

Image of FIG. 8.
FIG. 8.

Reynolds (Re) and magnetic-Reynolds ( ) numbers calculated from DRACO profiles at 1.5 ns. The ablation, critical, and quarter-critical surfaces are labeled for reference. The low point in Re at ∼60 m and in at ∼230 m represent where the fluid flow and advection velocity (including the Nernst effect), respectively, change directions.

Image of FIG. 9.
FIG. 9.

The fastest growing mode growth rate (short dashed) and wavelength (solid) profiles are shown near field onset times for (a) the spherical case at 0.8 ns and (b) the planar case at 1.5 ns. Simulated profiles were used with Equations (3) and (4) to generate the curves shown here and plotted as a function of (a) radial distance and (b) axial distance. An average wavelength was calculated at each time by averaging over this space and using the growth rate as a weighting factor. (c) The resulting average wavelength as a function of time for the planar (solid) and spherical (dashed-dotted) cases with the respective drives (dotted) shown at the bottom in arbitrary units. The inferred characteristic size of cellular structures determined from planar proton radiographs is also shown at .

Image of FIG. 10.
FIG. 10.

(a) Two synthetic images with eggcrate-like perturbations with diagonal peak-to-peak wavelength of 106 m and sinusoidal amplitudes of 10% (top) and 30% (bottom) of the mean. The corresponding AC spectra are shown for both images with the measured dominant scale size of the features as 113 ± 8 m. (b) Experimental radiographs from 1.4 ns (top) and 1.6 ns (bottom) with corresponding AC spectra. The earlier image does not show a peak in the AC spectra which suggests that there is no isotropic scale size in the image. Whereas, the radiograph at 1.6 ns clearly shows a peak at 185 ± 10 m indicating a dominant feature in the image.


Generic image for table
Table I.

Metrology of the four CH foil types used in these experiments. Initial ambient foil density was . Variations in thickness ( ), wavelength ( ), and sinusoidal amplitude ( ) are all . The value given for the eggcrate foil (3-D) is the diagonal peak-to-peak wavelength.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Instability-driven electromagnetic fields in coronal plasmasa)